Improvement by Benzoquinones of the Quantum Yield of Photoactivation of Photosynthetic Oxygen Evolution: Direct Evidence for the Two-Quantum Mechanism

Effects of eight differently substituted 1, 4-benzoquinones (BQs) on the quantum yield of photoactivation of oxygen evolution (reconstitution of the Mn cluster) were examined with wheat photosystem II (PSII) membranes depleted of the Mn cluster by treatment with 1.0 mM NH₂OH. Illumination with 10 flashes at 0.25-s intervals of the PSII membranes in the presence of 2.0 mM Mn²⁺, 20 mM Ca²⁺, and 1.2 M Cl^-restored 14% of oxygen-evolving activity destroyed by the NH2OH treatment. Among the benzoquinones tested, DBMIB (2, 5-dibromo-3-methyl-6-isopropyl-BQ) and tetramethyl-BQ did not enhance the activity recovery, but all the others doubled the recovery when present at their optimal concentrations during illumination. The order of effectiveness was tetrabromo-, phenyl-, and 2, 6-dichloro-BQs ≥ 2, 5-dichloro-BQ > tetrachloro-BQ > 2, 5-dimethyl-BQ, though the differences were small. This order reflects their efficiencies as electron acceptors of PSII. This finding, together with others, suggests that the enhancement of activity recovery results from rapid oxidation by the benzoquinones of the reduced form of the quinone acceptors in PSII, QA^-and QB^-, which cause loss of an oxidized intermediate through charge-recombination reaction with Mn³⁺. The flash-number dependence of the recovery of oxygen-evolving activity indicated that the activity was not restored after one flash but recovered significantly after illumination with two flashes and then further increased upon additional flashes. This provides direct evidence that the minimum quantum requirement for photoactivation is two.